gitk: Fix display of branch names on some commits

gitk: Fix display of branch names on some commits

Sometimes the code that divides commits up into arcs creates two
successive arcs, but the commit between them (the commit at the end
of the first arc and the beginning of the second arc) has only one
parent and one child.  If that commit is also the head of one or more
branches, those branches get omitted from the "Branches" field in the
commit display.

The omission occurs because the commit gets erroneously identified as
a commit which is part-way along an arc in [descheads].  This fixes it
by changing the test to look at the arcouts array, which only contains
elements for the commits at the start or end of an arc.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>